1) Field of the Invention
The current disclosure relates to polyamic acid and polyimide polymers with varying ratios of rigid monomers and solubilizing monomers exhibiting improved thermo-oxidative stability and processing characteristics.
2) Description of Related Art
The development of aromatic polyimides has a rich, international history that began in the 1950s with DuPont's “convertible polymers” research program, which aimed to produce processable polymeric precursors that could be converted to a final intractable polymer (U.S. Pat. No. 2,710,853). This research effort rapidly matured into the first commercial aromatic polyimide material prepared from pyromellitic dianhydride (PMDA) and oxydianiline (ODA), now trademarked as Kapton®, Vespel®, and Pyre-ML® for different applications. The PMDA-ODA polyimide possessed a combination of thermal durability, mechanical toughness, chemical resistance, and attractive electrical properties that was unlike any other polymer known at the time. Since that time, numerous other polyimide materials have been commercialized in applications ranging from electronics manufacturing to fiber reinforced structural composites to separations membranes.
Aromatic polyimide materials offer clear performance advantages over other polymers, especially in high temperature applications where thermal durability is critical; however, processing of these materials is characteristically difficult. Furthermore, the more thermally stable a polyimide composition is, generally the more difficult it is to process. For example, the polyimide prepared from the most readily-available and reactive of common monomers, pyromellitic dianhydride (PMDA) and 1,4-diaminobenzene (PDA), has not been highly commercialized because the polymer is too brittle to be useful in most applications. Further, the polyimide prepared from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (BPDA) and PDA, originally developed by Ube Industries and produced under the trademark Upilex®, has been reported to be one of the most thermally stable and chemically resistant materials of common commercial polyimide structures.
The exceptional thermal stability of the BPDA-PDA polyimide results from the very linear, rigid, and fully aromatic nature of the polymer. Unfortunately, the same intrinsic properties that lend high thermal stability and chemical resistance to the material also cause it to be less mechanically tough, anisotropic and partially crystalline, and difficult to apply to substrates without the use of adhesion promoters. Even the polyamic acid precursor to the BPDA-PDA polyimide is of low solubility and difficult to dissolve in conventional solvents at concentrations much above 10 wt % with useful viscosity.
The demand for polyimide materials with greater processability has led to a wide body of research and development in both academia and industry. Commercial polyimide structures were developed with alternative monomers such as 4,4′-oxydiphthalic anhydride (ODPA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4′-oxydianiline (ODA) to make polyimide materials with increased mechanical toughness and improved processability compared to BPDA-PDA. The ether-containing monomers, ODPA and ODA, have the added benefit of increasing the hydrolytic stability of the polyimide. Monomers with substituted aromatic structures have been employed to disrupt polymer chain packing and improve solubility as well as affect other properties of interest such as optical transparency and physical permeability. U.S. Pat. Nos. 5,071,997 and 5,310,863 describe the use of trifluoromethyl-substituted diamines to increase isotropy and improve mechanical strength in BPDA-PDA and other polyimides.
The melt-processable polyimide resins developed by the National Aeronautics and Space Association (NASA) were a breakthrough in polyimide technology, opening the way for resin-transfer molded polyimide parts. The development of melt processable polyetherimides, such as Ultem® developed by General Electric Plastics, facilitated the manufacture of extruded, thermoformed, blow molded, and injection molded polyimide products, as well as fiber reinforced structure resins and pre-impregnated composite materials.
While all of these advances have enabled a large field of use for polyimide materials, applications such as harsh environment fiber optics continue to drive demand for materials with higher thermal stability and more facile processing. Prior to this disclosure, no polyimide material has been developed that possesses thermal stability equivalent to that of BPDA-PDA without its processing limitations; thus a design approach to polyimides with greater ease of use as well as exceptional thermal durability and chemical resistance is the aim of the present disclosure.
In order to fully address the need for polyimide materials with greater processability, the thermal imidization process, which is common and critical to most applications, must be considered. Conventional polyimide materials such as BPDA-PDA and PMDA-ODA are regarded as thermoset polymers and are also insoluble in all organic solvents; these inherent properties dictate that they must be cast from solutions of their more processable precursors, polyamic acids. As a result of this, manufacturing processes for polyimide films or coated substrates must provide sufficient thermal energy to evaporate the casting solvent, convert the polyamic acid to the polyimide, and remove the water that evolved as a product of thermal imidization. To fully convert the polymer, processing temperatures must exceed the glass transition temperature of the polyimide, more than 450° C. Any residual polyamic acid left in the final product is deleterious to its performance in most applications due to the poor thermal and chemical stability of the polyamic acid bond, which depolymerizes slowly at room temperature and rapidly at elevated temperature or in the presence of water.
U.S. Pat. No. 5,714,196 discloses an approach to a low temperature curing polyimide coating, where a polyamic acid polymer solution is cured at temperatures no greater than 300° C. on a low glass transition temperature optical fiber. While this approach does not utilize the polymer design of the present disclosure nor does it guarantee a fully imidized coating like a pre-imidized coating solution as disclosed herein, it demonstrates the advantage of a low cure temperature to form a unique fiber configuration. Additionally, the authors show another advantage of a soluble polyimide coating—the polyimide coating could be stripped from the fiber with acetone.
Accordingly, it is an object of the current disclosure to provide a novel approach to polymer design in order to provide polyimide materials that can be processed from pre-imidized solutions yet possess thermal stability and chemical resistance sufficient for use in harsh environments.
It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all its respects, to every aspect of this invention. As such, the preceding objects can be viewed in the alternative with respect to any one aspect of this invention. These and other objects and features of the invention will become more fully apparent when the following detailed description is read in conjunction with the accompanying figures and examples. However, it is to be understood that both the foregoing summary of the invention and the following detailed description are of a preferred embodiment and not restrictive of the invention or other alternate embodiments of the invention. In particular, while the invention is described herein with reference to a number of specific embodiments, it will be appreciated that the description is illustrative of the invention and is not constructed as limiting of the invention. Various modifications and applications may occur to those who are skilled in the art, without departing from the spirit and the scope of the invention, as described by the appended claims. Likewise, other objects, features, benefits and advantages of the current disclosure will be apparent from this summary and certain embodiments described below, and will be readily apparent to those skilled in the art. Such objects, features, benefits and advantages will be apparent from the above in conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom, alone or with consideration of the references incorporated herein.